An interesting subject!!
I think that some sets are very rare, yes! As said in other posts:
Old AIRFIX...(Zoo, Farm Animals...Cosmonauts, Civilians from Station accessories, etc...)
I have too "Fort Apache"...
Old ATLANTIC (I have 2 complete boxes: Apache Camp and Cowboys, and The wagon Train (complete in sprue!) which is really rare!!!
NEXUS or even AIRFIX can't release old sets because moulds are lost!
(Maybe, a lot of them in the Golf War...
)
Sometimes, sets could be found on e-bay, but price are good for collectors!!
Eric: it's the first time I see The "Thierry La Fronde" set!!!
not a lot of poses! (8 if my eyes are good enough!
)